When precipitation falls on the surface of the earth, the water may quickly run into streams and eventually be carried to the sea. It may also be held in lakes or oceans, sometimes for many years. Or it may soak into the ground where it might be held for a few years to thousands of years, depending on how deep it goes into the earth. Much of the precipitation that falls is evaporated again almost immediately. Eventually the water that is held in oceans, lakes and the ground again becomes part of the cycle.

If you consider the earth as a whole, the water cycle is a "closed" cycle; that is, no water is gained or lost. But if you consider a particular location on earth, you may see "wet" years and "dry" years when there is more or less rain than usual. In the polar regions, there may be accumulation or loss of ice and snow, depending on the earth's temperature. Long-term shifts in the earth's temperature has led to ice ages in the past; at present the earth is in a warming trend, with ice and snow in polar regions and in glaciers being transformed to liquid water in the earth's oceans. If this trend continues, coastal areas could be threatened by sea level rise.

A useful resource for further information can be found at http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml